MEDIUM: filters: eval the legacy backend filters first on the response path

MEDIUM: filters: eval the legacy backend filters first on the response path

In the legacy filter mode, the backend filter configurations are now
prepended to the response filter list when they are attached to a stream,
preserving their own response order, so a response is evaluated by the
backend filters before the frontend ones. The request path is unchanged.

This mirrors the behavior of the new filter mode on the response path: a
response travels in the opposite direction of a request and reaches the
backend filters first.

The regression tests relying on the response evaluation order of the legacy
mode were updated accordingly, including the mixed cases of the stream-order
test where a legacy frontend uses a instance-mode backend and the other way
around. The filters API documentation was also updated: the frontend filters
are called first on the request path, the backend ones on the response path.